Magneto-based ignition systems are well known and are often used with internal combustion engines in applications where batteries are not practical. Magnetos are robust devices that are typically highly reliable. As such, magneto-based ignition systems have been historically used with internal combustion engines for aircraft applications. In a typical magneto-based ignition system for an aircraft internal combustion engine, redundant ignition systems are employed for safety purposes. Also, with safety in mind, the magneto-based ignition systems for aircraft are typically mechanically timed to ensure highly reliable operation. In this connection, it is not uncommon for small aircraft to experience malfunctions of their electrical systems. Because of the obvious need to ensure high reliability of the ignition systems for internal combustion engines in aircraft applications, such ignition systems have historically avoided electronic ignition control mechanisms for advancing and retarding spark timing, even though such electrical control devices are commonly used in automotive applications.
Because magneto-based ignition systems in aircraft applications employ mechanical linkage to time the spark events, the timing of the spark event for each piston is at a fixed advance with respect to the "top-dead-center" (TDC) position of the reciprocating piston. The advance is typically selected for optimum performance under take-off conditions. Unfortunately, during different parts of a flight, the engine is operating in different conditions. Therefore, the advance and total energy of a spark event that provides the most efficient combustion and energy conversion varies during the flight. In the past, the small aircraft industry has sacrificed engine performance in order to ensure safety by maintaining the fixed mechanical advance of the spark event for all engine operating conditions. As a result of the fixed mechanical advance of the magneto-based ignition systems for small aircraft, the engines operate at less than optimum fuel economy and exhaust more pollutants.
In one example of a previous attempt to provide controlled advance timing of the spark event in a magneto-based ignition system, U.S. Pat. No. 4,624,234 to Koketsu et al. describes an electronic circuit for controlling the timing of the spark event and a mechanism for defaulting to a mechanical timing when the regulated voltage supply for the electronic circuit is inadequately regulated. In this system, however, the sole source of energy for the spark event is the rotating magnet of the magneto. Unfortunately, the power curve of the magneto is mechanically fixed and the mechanical advance for the ignition is usually selected to occur at the peak of the magneto's power curve. Therefore, changing the timing of the spark event relative to the mechanical setting results in a reduction in the energy of the spark event.
Other attempts have been made to employ both the energy from the rotating magnet of a magneto and the energy from a battery in the electrical system of the engine. For example, U.S. Pat. No. 1,074,724 discloses providing energy to the primary coil of a magneto by way of both the conventional rotating magnet of the magneto and from a battery. By appropriately synchronizing the timing of the ignition system with the rotating magnet, the patent provides for the spark event to occur only during the positive portions of the alternating positive and negative voltages impressed on the primary coil by the rotating magnet, thereby ensuring the energy from the battery complements the energy from the rotating magnet when the spark event occurs. Only conventional mechanical breaker points are used in this ignition system, resulting in a mechanically fixed ignition timing for all engine operating conditions. Therefore, over much of its operating conditions, the engine operates inefficiently.